<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .a {
        color: red;
      }
      .b {
        font-size: 3rem;
      }
      .c {
        border: solid;
      }
    </style>
  </head>
  <body>
    <div id="app">
      <div>
        <h3 class="a b c">this is class style</h3>
        <h3 v-bind:class="[activeA,activeX?'b':'c']">this is class style</h3>
        <button @click="setActiveB">activeB</button>
      </div>
    </div>
    <script src="./vue.js"></script>
    <script>
      new Vue({
        el: "#app",
        data: {
          activeA: "a",
          activeX: true,
        },
        methods: {
          setActiveB() {
            this.activeA = "f";
            this.activeX = false;
          },
        },
      });
    </script>
  </body>
</html>
